Looking Up: Once in a while one blows up

Stars are not as steady as they first appear. Constellations of stars remain our faithful friends, returning night after night or year to year. For the most part the stars never seem to change.

Many stars are known to vary periodically in brightness, and it is interesting to track them and plot their estimated magnitude on a chart, with the magnitude scale on the vertical side and he dates or even hours on the horizontal line or bottom of your chart. You end up with a graph showing how the star peaked in brightness and slowly curved downward toward minimum light.

There are many classifications of variable stars. Some change in brightness because they have a dimmer star revolving around it, periodically eclipsing. Others have within their nature something that causes the star to pulsate and vary in output.

None of these, however, are as startling as a supernova.

These are extremely rare events where a star literally explodes in a fury unmatched by another star you have likely ever seen. On a few occasions in history, mankind has marveled in wonder and even fear at the sight of a suddenly brilliant star where no star had been known to exist, outshining all the stars in the sky and even rivaling the Moon.

Astronomers tell us that the supernova likely was a "type I-a"- a white dwarf star that gathered too much stellar material from a companion star, and detonated.

Brilliant supernovae occurred in 1054 in Taurus the Bull; 1572 in Cassiopeia; and 1610 in Scorpius. No one knows when the next one will occur but the Milky Way Galaxy is long overdue. Astronomers have observed many of them in other galaxies and can estimate how often they happen. A supernova becomes the brightest star in the entire galaxy and flashes its light across the Universe.

They are expected to occur on average, once a century, but the last one known in the Milky Way was more than 400 years ago.

Occasionally, backyard telescope enthusiasts have seen- and even discovered- a far off supernova, in another galaxy. Normally the individual stars in a distant galaxy won't be resolved, but if a supernova occurs, it appears for a few weeks in your telescope as a faint star superimposed on the hazy galaxy, the farthest individual star you likely have ever seen.

After a supernova has faded away, they leave behind an expanding, nebulous cloud visible in telescopes. The Crab Nebula in Taurus is a familiar backyard telescope target and is what is left from the 1054 explosion.

First quarter Moon is on May 18. Saturday evening, May 11, be sure to look for the beautiful two-day old crescent Moon, just below brilliant Jupiter in the west.